Detachment folding in the Southwestern Tian Shan–Tarim foreland, China: shortening estimates and rates

Abstract

Geologic observations of the Chinese Tian Shan foreland reveal active, contractional deformation along the entire margin. To quantify the amount of shortening and understand the structural style, we mapped the stratigraphy and structure of four folds expressed at the western end of the foreland, north of Kashi. In this region, upper Tertiary through Quaternary sediments are conformable, but an abrupt transition from parallel to unconformable bedding in the uppermost strata suggests a transition from pre-fold strata to strata deposited on a growing fold. The folds have very steep (60–90°) limbs and are box-like to isoclinal, suggestive of detachment folding. Total north–south shortening across the center of the region is >9 km, of which 5–7 km occurred in the Kashi–Atushi fold system. Shortening estimates determined with excess area methods for individual folds decrease from a maximum of 6.8 km in the northwest to a minimum of 0.7 km in the southeast. Timing derived from a paleomagnetic study shows that the transition to syn-folding strata occurred ∼1.2 Ma in the middle of the study area, resulting in an average shortening rate for the Kashi–Atushi fold system of ∼5 mm/yr if folding was coeval. The shortening rate is high compared with foreland deformation east of the study area, suggesting that the regional stresses or response of the foreland stratigraphy are unique to the Kashi–Atushi fold system kinematics.